Rotary material handling valves such as those disclosed, for example, in Starrett U.S. Pat. No. 3,273,758 and Starrett U.S. Pat. No. 3,750,902, are used for transferring particulate material into a pressurized environment at raised temperature. For example, such valves are used for feeding raw materials such as pulp wood chips and sawdust into a container of hot pulping liquor in the process of manufacturing paper. On a receiving side of such a valve, the chips are fed to the valve at temperatures near ambient atmospheric temperatures and at atmospheric pressure. The valves must deliver those materials into a container where temperatures and pressures are elevated significantly. It is desirable to feed the raw materials into the processing container continuously at high volume rates, but with a minimum loss of pressure from within the processing container.
Valves of the type with which this disclosure is concerned require close clearances between a tapered rotor and a surrounding housing when the valve is fully warmed up and operating in a stabilized continuously operating condition. Such valves may have rotors varying in diameter from 35 inches to over 40 inches along the length of the rotor.
Previous practice in the use of such rotary valves has included placement of the axis of rotation of the rotor of the valve in an eccentric location with respect to the tapered bore in the surrounding body of the valve when the valve is assembled at normal atmospheric temperature, and the valve has thereafter been operated with frequent adjustment during a break-in period during which the rotor and the opposing surfaces of the housing have worn each other until the rotor has become seated around the entire periphery of the housing with the desired close clearance so as to minimize leakage of gas under pressure from the processing container into which raw material is being fed by the valve.
The wear-in period includes axial movement of the rotor with respect to the body by means of an adjustable thrust bearing arrangement, and a considerable amount of material has to be worn away from the mating surfaces of the rotor and housing before the desired close fit is attained in a tapered rotary feed valve manufactured in accordance with Starrett U.S. Pat. No. 3,750,902. Since the rotor has to be moved axially into a position in contact with the housing, so that wear will occur where necessary, the power required to rotate the rotor during break-in is higher than is usual once a valve has been broken in, and leakage of pressurized gas or steam from the processing container occurs, wasting heat, during the break-in period.
The required wear-in period for such valves has been many hours long, often as much as two weeks.
As stated in Starrett U.S. Pat. No. 3,750,902, it has commonly been believed that an offset of the axis of rotation of the rotor was desired to accommodate elastic flexure of the rotor shaft and to accommodate take-up of clearances in the bearings themselves, so that the rotor would essentially be centered in the housing, with minimal clearance between mating surfaces of the rotor and housing, during operation of the valve. Excessive leakage into the end-bells had resulted from the misfit of the rotor in the valve body with the rotor bearings centered. The real reason the bottom sector was not sealing fast enough was the misfit at the rotor-to-body seal areas in the lower sector in a new valve. This was a result of the thermal profile of the body in a stabilized hot valve. The downwardly eccentric displacement of the rotor had the effect of reducing the amount of plugging in the ends, etcetera, long enough for the rotor to wear-in until the rotor was effectively “lapped” in to fit the bore at hot equilibrium. It should be noted, the problem of leakage was particularly problematical when users started using sawdust for furnish in the early 1970's.
The downwardly eccentric adjustment of the rotor axis caused the lower sector of the bore to wear-in to a close fit first. Quickly establishing a seal in the lower sector was paramount, because without it fines—especially sawdust—escaped past the seal at each end of the rotor in the bottom sector, causing end-bell plugging, excessive gas bypass into the inlet, and troublesome startups.
In the valve manufacturer's manuals, regrind was advised when the user finds the valve liner thickness varies 0.020″. The user would find this condition every time the unit was taken apart, as a result of the wear during break-in because of the prescribed downwardly eccentric rotor shaft location.
Two primary causes may contribute to change in shape in a high pressure rotary valve: thermal profile variations and elastic deflection because of pressure differential across the valve. Both are related to properties of the materials of which the rotor and the housing are manufactured. It has been calculated that the strain of the housing, or body, of the valve because of pressure differential would be about seven millionths (0.000007) of an inch (insignificant for a Bauer rotary valve, the type of valve disclosed in the Starrett patents), far less than the offset, eccentrically downward placement of the axis of rotation of the rotors according to the manufacturer's recommendations for those valves.
As for possible deflection of the shaft, bending in response to pressure between the outlet and inlet sides of the valve, based on 150 pounds per square inch pressure differential across the valve, the maximum deflection in bending, determined by a finite elemental analysis of the rotor, is also much less than the recommended downward eccentric offset location of the shaft. Elastic deflection therefore does not require the use of an eccentric offset location of the axis of rotation of the rotor.
The real issue is one of thermal profiles in the materials of the housing and rotor to account for the shape and size of the housing and rotor in a condition of equilibrium once the valve has been in operation long enough to achieve its operating temperature during continuous operation.
What is desired, then, is an improved way to manufacture or overhaul and assemble such a rotary valve so that the time required for break-in is shorter and the valve is operable thereafter for a longer time.
In operation of a rotary feed valve, uniform heating of the rotor occurs because the rotor rotates over the heat source, the pulp digester or other container with which the rotary valve is used, because of the higher temperature of the materials under pressure within the container on which the valve is mounted. The housing, or body, of the high pressure rotary valve is bolted to the container, which is the heat source, and is therefore subject to a thermal gradient from the bottom of the valve toward its top. Additionally, as the temperature of the rotor increases, its initial taper angle increases, because the large end of the rotor, with a larger radius, expands by a larger distance than does the smaller end of the rotor.
Because the upper portion of the body of the valve never is heated to as high a temperature as the bottom portion, the internal radius of the body does not increase as much in the upper sector of its bore as in the lower sector.
Initial operation of such a high pressure rotary valve when new or refurbished to factory specifications requires the body bore to be worn into a shape matching that of the rotor, so that the valve minimizes pressure loss when it has reached its equilibrium temperature distribution after operating for a long enough time.
It takes about eight hours for a tapered high pressure rotary feed valve of the size used in pulp processing to reach thermal equilibrium under continuous operation. Ideally, when starting such a valve one would give the valve at least three to four hours in which to stabilize. With a shorter time allowed for starting operation of the valve the thermal gradient between the top and bottom of the valve becomes larger, and the distortion of the shape as a result of unequal temperatures is greater.
The rotor stays round; it wants to seal all around on its circumference, on each end. At the same time the upper sector of the body bore, frustoconical when shaped at ambient temperature, takes on an oval section profile when the valve body reaches thermal equilibrium in operation. The non-uniform temperature profile thus prevented the rotor from sealing at the seal edges in the lower sector, because the rotor grows, radially, from heating—more than the surrounding body—and had to be pulled back axially to the last contact position, which is in the cooler upper sector of the body. Pulling the rotor back axially (thereby increasing the rotor-to-body clearance at the bottom) to compensate would make the leakage into the end-bell cavities worse.
The rotor contact first taking place at the top of the bore apparently led Bauer to surmise that the rotor had been deflected and at least to determine the rotor needed to be moved into the lower sector to mitigate the leakage (at the bottom) into the end-bell cavities, during the break-in or wear-in period.
In accordance with the present disclosure, the rotor axis of rotation should be located on the primary bore axis. As defined in the claims which form a part of this disclosure, the interior mating surfaces of the housing, or body, of such a rotary valve should be shaped when at normal ambient temperature during manufacture or overhaul to have a non-circular internal section shape that will become circular when the valve reaches its normal stabilized operating temperature profile, so that the rotor will fit the body with relatively little break-in wear being needed.
The body or housing can thus be prepared so that a minimum amount of break-in time is required for the rotor and body bore to wear in to fit against each other when the valve has reached equilibrium at its operating temperature and temperature distribution.
In general terms, this is accomplished by shaping the interior surfaces of the body bore of the housing, with the housing at ambient temperature, so that with the rotor located with its axis of rotation in the designed location the rotor clearance in the upper half or sector of the bore is greater than that in the lower sector of the bore, with the difference in clearance being equal to the difference in thermal expansion of the bore in the top sector or half of the body, relative to the thermal expansion of the bore in the bottom sector or half as a result of the different temperature increases of the different portions of the valve body between ambient temperature and operating temperature of the valve.
In one embodiment of the method disclosed herein a valve body may be prepared by first cutting or grinding the body to form an initial bore with interior surfaces symmetrical about the axis of rotation of the rotor, and then removing additional material from the interior of the valve body to form an enlargement of the initial interior bore, of the same size and shape as the initial bore, centered about an axis parallel with the axis of rotation of the rotor but displaced toward the inlet side of the body by a distance related to the difference in temperatures between the inlet side and outlet side of the valve in stabilized operation.
As a further refined variation, the bore of the body of the valve can be adjusted to a slightly different cone angle to account for the slightly greater enlargement of the larger end of the rotor as it is heated to operating temperature.
The foregoing and other features of the invention will be more readily understood upon consideration of the following detailed description of the invention taken in conjunction with the accompanying drawings.